math
posted by iris .
john and mary are working on a job together. if john does it alone, it will take him seven days, while mary can do it alone in five days. how long will it take them to do it together?

john rate= job/7hrs
mary rate=job/5 hrs
combined rate= job/7hrs+job/5hrs= (5+7)/35= 10/35 job/hr
so, time= howmuch/rate= 1job/(10jobs/35hrs)= 3.5 hrs 
Or, if you use base 10, where 7+5=12, that would be 35/12 hours, or 2 hours 55 min
The 3.5 hours caught my eye, because that would be the time taken by 2 people who take 7 hours each.